The invention relates generally to synchronizing a clock to a system time and, more specifically, to synchronizing a clock to a system time using multiple time sources.
In some multi-node communications networks, it is important that each node is synchronized with the system time. This is particularly true in wireless networks, where each node is responsible for sending data packets to other nodes or to destination devices. For example, in a wireless network that uses a time division multiple access (TDMA) communications protocol, a node must be synchronized with the system time so that it can transfer data packets during the appropriate time slots. If the node is not synchronized with the system time, the node may miss sending the data packets within the correct time slots, resulting in lost data. The inability to maintain synchronization with the system time can have significant consequences, such as a severe node failure or an inability to establish or maintain communications channels, for example.
Some prior art systems use the Global Positioning System (GPS) as a source of system time. These systems incorporate a GPS receiver within each node that has a clock to be synchronized. The GPS receiver receives signals from the multiple GPS satellites, and the system uses those signals to determine system time and synchronize its clocks. Some of these systems use a carrier frequency signal to calibrate the GPS receiver""s local oscillator. Such a system is described, for example, in U.S. Pat. No. 5,841,396, entitled xe2x80x9cGPS Receiver Utilizing a Communication Link,xe2x80x9d issued on Nov. 24, 1998.
One disadvantage to using GPS for time determination is that the GPS receiver must have a direct line of sight with multiple GPS satellites in order to achieve an acceptable level of accuracy. Generally, the more satellites that are in the field of view of the GPS receiver, the more accurate are the time determinations. For example, under current levels of selective availability imposed on civilian GPS signals, the xe2x80x9cthree sigmaxe2x80x9d accuracy is roughly 100 nanoseconds divided by the square root of the number of GPS satellites tracked by the receiver.
On the other hand, the fewer the GPS satellites signals received (e.g., due to GPS satellite failures or obstructions along the line of sight between the receiver and the GPS satellites), the less accurate the time determinations will likely be. Therefore, the accuracy of the time determinations is highly susceptible to GPS satellite visibility, making GPS a non-robust system for some applications.
What are needed are apparatus and methods that provide a highly available and accurate, synchronized clock. Further needed are apparatus and methods that can enhance the accuracy of a GPS reference signal in order to provide a more accurate clock than is possible with prior art apparatus and methods.